Signal detecting circuit



Jan. 5, 1960 w. sAEGER 2,920,239

SIGNAL DETECTING CIRCUIT Filed Jan. 30, 1957 IN V EN TOR.

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\ M +6054 M ArzvRzmY United States Patent O SIGNAL DETECTIN G CIRCUIT Waldemar Saeger, Pennsauken, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application January 30, 1957, Serial No. 637,161

4 Claims. (Cl. 315-170) This invention relates to signal detecting or indicating circuits, and particularly to such circuits that employ devices having a hysteresis characteristic.

One device having a hysteresis characteristic is a gas discharge tube. Such a tube has a firing potential that is substantially greater than its extinction potential. As a result of this hysteresis, a gas tube such as a neon diode is lighted by a potential in excess of its ring (or ionizing) potential, and, once lighted, remains lighted at lower potentials that are in excess of the extinction (or deionizing) potential. Similarly, once extinguished, the neon diode remains extinguished at potentials in excess of the extinction potential but less than the ring potential. Such a tube may be used to indicate two voltage levels by its lighted and extinguished conditions, respectively.

One form of such neon diode, the type NE51, may have as much as ten or twenty volts spread between the firing and extinction potentials. Consequently, this tube may not be used directly for indicating two voltage levels that differ by an amount substantially less than the difference between the tiring and extinction potentials, for example, a voltage of approximately one volt. These neon tubes have been limited in their utility by considerable variation from tube to tube in the ring and in the extinction potentials. Another limitation in utility results from the characteristics of the tubes changing substantially as they age.

It is among the objects of this invention to provide:

A new and improved indicator circuit using gas discharge tubes;

A new and improved gas tube indicator circuit for indicating small voltage changes;

A new and improved gas tube indicator circuit that is reliable in operation notwithstanding variations in tube characteristics.

In accordance with this invention, a hysteresis device, such as a gas discharge tube, is connected to receive voltages to be indicated and also a bias voltage that includes a direct voltage component and an alternating voltage component. The received and bias voltages are combined so that in ordinary operation the device is either turned on or turned oif.

The foregoing and other objects, the advantages and novel features of this invention, as well as the invention litself both as to its organization and mode of operation, may be best understood from the following description when read in connection with the accompanying drawing, in which like reference numerals refer to likeparts, and in which:

Figure 1 is a schematic circuit diagram of an indicator circuit embodying this invention;

Figure 2 is an idealized graph of the voltages applied tothe indicator tube of Figure l;

vFigure 3 is a schematic circuit diagram of another embodiment of this invention;

Figure 4 is an idealized graph of voltages that may appear across the indicator tube of Figure 3; and

2,920,239 atented Jan. 5, 1960 ICC Figure 5 is a schematic circuit diagram of a gas tube indicator circuit embodying this invention used to indicate the states of a iiip-op circuit.

In Figure l, a gas tube indicator circuit embodying this invention is shown. This circuit includes a gas diode 10, which may be a neon tube. An input terminal 12 is connected through a current limiting resistor 14 to one electrode l5 of the gas tube 10. The other electrode of the gas tube 10 is connected through a secondary winding 16 of a transformer to an adjustable tap 18 of a potentiometer 20. One terminal of the potentiometer 20 may be connected to the positive terminal of a directvoltage (DC.) source, and the other -terminal may be connected to a common return shown by the conventional ground symbol. The primary 22 of the transformer (which may be a filament transformer) receives van alternating (AC.) voltage from a source 24, which source 24 may be a 60-cycle line.

The input terminal 12 receives a voltage that varies in amplitude from a source 26 of variable voltage. By way of example, the voltage from the source 26 is shown as a step voltage going from +1 volt to -l volt with respect to ground. By way of illustration, the gas tube 10 is assumed to have a tiring potential of volts and an extinction potential of 60 volts. The potentiometer tap 18 is adjusted to provide a direct-voltage approximately midway between the tiring and extinction potentials, that is, a D.C. level of 65 volts. The voltage supplied by the transformer secondary 16 is approximately the difference between the ring and extinction potentials, which is, for the assumed tube characteristics, 10 volts peak-topeak. Thus, the combined voltage supplied to the tube electrode 17 by the transformer secondary 16 and the potentiometer 20 together is an alternating voltage that varies 5 volts from the D.C. level of 65 volts. This voltage waveform supplied to the tube electrode 17 is shown in broken lines in Figure 2 and is referenced by the numeral 30.

In operation, assume that the tube 10 is extinguished initially, and that the voltage at the input terminal 12 is +1 volt. Under these circumstances, the voltage across the electrodes 15 and 17 of the tube 10 varies between 59 and 69 volts as shown by the waveform 32 (Figure 2). Thus, the voltage across the tube 10 remains less than the ring potential of the tube 10, and the tube remains extinguished. If the input terminal voltage is changed to -l volt, the combined voltage across the tube 10 varies between 61 volts and 7l volts as shown by the waveform 34. When the voltage across the tube 10 rises above the tiring potential of 70 volts, the tube tires; this ring occurs within a cycle of the alternating voltage supplied by the source 24. Once tired, the tube 10 remains in the lighted condition, because the voltage across the tube remains above the extinction potential of 60 volts. When the input terminal voltage rises again to -l-l volt, the voltage across the tube 10 follows the waveform 32, and falls below the extinction potential within one cycle of the A.C. voltage. At that time, the tube is extinguished, and remains extinguished until the input voltage drops again to -1 volt.

Thus, by means of the combined direct and alternating bias voltages supplied by the potentiometer 20 and the transformer secondary 16, the circuit of Figure l indicates small input voltage changes that are substantially less than the difference between the firing and extinction voltages of the tube 10. The tube 10, once iired (or extinguished), remains red (or extinguished) as long as the corresponding input voltage is supplied. Thereby, the tube 10 provides an essentially fully-lighted or a fullyextinguished indication corresponding to the input voltage received. The variable voltage source 26 may be any device that provides different voltages; for example,

the source 26 may` be the indicating arm Vof a bridge circuit or an electronic switch circuit.

The frequency of the A.C. source 24 may have a considerable range, dependingon the response time of the neon tube .10 and the time of duration of a voltage step (the display time of the voltage step) at the input terminal 12: The maximum frequencymust be such that within a cycle the voltage is either above the ring potential or below the extinction potential for a suicient time for '.the tube to change state, that is, the maximum frequency should be not greater than the response frequency of the tube. The minimum frequency is significant if the voltage at the terminal 12 is in the form of a momentary voltage step, positive or negative with respect to ground. Under such circumstances the frequency should be such that a complete A.C. cycle occurs during the display time of the step.

In Figure 3, a neon-diode 40 is used to indicate the state of conduction or non-conduction of an electronic switch in the form of a transistor 42. One electrode 44 of tube 40 is connected through a current limiting resistor 46 to the collector 48 of the transistor 42. The other electrode 50 of tube 40 is connected through a choke coil S2 to a direct voltage source 54, shown as a battery, the other terminalv of which is connected to ground. An A.C. source 56 is coupled by a capacitor 58 to the electrode 50. (Alternatively the A.C. coupling may be a transformer as described above with respect to Figure l, which arrangement would not use the choke 52.)

In the transistor circuit, the collector 48 is connected through a collector load resistor 60 to the negative terminal of a source 62 of operating potential, the positive terminal of which is returned to ground. The emitter 64 is also connected to ground. An input terminal 66, which receives a step voltage 68 with respect to ground, is connected through a resistor 70 to the base 72 of the transistor 42. The base 72 is connected through a biasing resistor 74 to the positive terminal of a source of biasing potential 76, the negative terminal of which is returned to ground. The parameters shown in Figure 3 are illustrative of an operative embodiment of this invention and are not construed as a limitation of the scope of this invention.

It is assumed that the neon tube 40 may have a range of firing and extinguishing potentials. This range of characteristics may be due to the variations that usually occur in manufacture of such tubes, or they may be due to the changes that occur with aging of a tube. By way of illustration, it is assumed that the maximum of the range of firing potentials that the tube 40 may have is 82 volts (as indicated by a dotted horizontal line in Figure 4). The minimum lfiring potential is assumed to be 65 volts-(as shown in Figure 4); the maximum extinction potential, 66 volts; the minimum extinction potential, 52

volts; Thus, the hysteresis range, that is, the difference between the maximum tiring potential and the minimum extinction potential, of the tube 40 is 30 volts, for the assumed values, which range has been encountered in practice. Actually, a particular tube 4t) has a particular firing potential and a particular extinction potential, both Within the hysteresis range, at any time of circuit range.

In the transistor circuit, the collector-emitter operating voltage level (supplied by the source 62) may be chosen at a suitable value substantially below the breakdown voltage. For the transistor type indicated in Figure 3, the usual breakdown voltage may be expected to be less than the 30-volt hysteresis range that is assumed. For conservative operation, the voltage across the collectoremitter path maybe 18 volts, and -18 yvolts is used as the collector voltage for the illustrated circuit.

The peak-tolpeak; amplitude-of vthe' A.C. ,voltage sup plied bythe source 56 to.;the electrode. 50.is vchosen toY have a -value`that, together with the collector voltage, supplies the maximum range of hysteresis voltage (the difference betweenthe maximum tiring potential. and the substantially ground potential.

istics are respectively supplied. For the illustrative values considered, this direct biasv voltage from the source 54 is 58 volts.

In operation, when a relatively 10W voltage input level is applied to the input terminal 66 to render the transistor 42 conductive, the voltage at the collector 48 is Under these conditions, the voltage across the tube 40 is the sum of the direct bias voltage from .the 'source 54 andthe A.C. voltage supplied through the condenser 58; this combinedvoltage is shown in idealized form as the waveform in Figure'` 4. The voltage applied to the tube electrode50, which is also substantially the voltage across the tube40, varies approximatelyy between 52 volts and 64 volts, as. shown by waveform 80. Thus, this voltage acrosslthe 'tube 40 is always at least equal to the Iminimum extinguishing potential, and the tube 40 is extinguished.

When a relatively high -level of input voltage is applied to the input terminal 66, the transistor is cut off, and the collector voltage falls toward the -18 volt level of the source 62. With the collector 48 at -18 volts, the voltage across the tube 40 takes the form vof the idealized graph 82 (Figure 4). That is, the voltage across the tube 40 varies approximately between 82 volts and 70 volts. Thus, within one cycle of the A.C. voltage 82, atleast the maximum tiring potential level of S2 volts acrossl the tube40 is attained (unless the tube 40 lires at a lower potential level) and the tube 40 fires. Once red, the

tube remains tired, because the voltage across the tubeis again rendered conductive. Under such circumstances,-

the collector voltage is substantially atV groundpotential,

and the voltage across the tube 40 again takesthe form` of the waveform 80. Within one cycle of that Waveform Si), the voltage across the tube falls to the minimum-ex-' tinction potential level and the tube 40 is extinguished. The tube remains in the extinguished condition until the transistor 42 is again cut off, and the collector voltage decreases, as described above.

Thus, with the circuit of Figure 3, a neon Vtube `40 may be used to indicate the conductive state of a transistor 42, with the voltage swing at a transistor-electrode being substantially less than the range of .hysteresis of the neon tube 40. The transistor electrodervoltage may be substantially less than the breakdown voltage of the transistor, so that it may be operated conservatively.

This transistor electrode voltage, together with the.A;C.V

bias Voltage, assures properA operation-over a substantial range of neon tube hysteresis. Y

The waveforms 80 and 82 in Figure 4 respectively represent two dynamic conditions of operation'of the tube 40; that is, the waveform 80 represents the periodic voltage across the tube 40 for the condition-ofthetube being completely extinguished during each cycle ofthe alternating voltage. form 82 represents the periodic voltage across the sef ries combination of the resistors 60 and 46 Vand the tube 40. As may be. seen .in VFigure V4, .the maximum extinc tion potential and the minimumY tiring potential, in .the range of tube characteristics that is assumed, are notjineluded within the dynamic:y ranges of potentiaLacroSs the tuhe,..orY across. the series network including the tube, for either the red or extinguished conditions. One-'reason for such a. condition isA thatA the .maximum extinction potentialis greater. than the minimum ringpotential-for- In the red condition the Wavrethe assumed range of variations. These latter potentials should be respectively outside of the fired and extinguished dynamic ranges if the tube 40 is to be completely extinguished or completely tired over each cycle.

By using a larger amplitude alternating voltage, say, a peak-to-peak amplitude of about 16 volts instead of 12 volts, the tube 40, once fired, would continuously remain in the fired condition for each A.C. cycle. However, the voltage across the tube 40, when it should be in the extinguished condition, would exceed the minimum firing potential level of 65 volts during a small portion of each A.C. cycle. Thus, the tube 40 may actually be tired for about a quarter cycle when it should be in the extinguished condition. This momentary tiring of the tube 40 provides a dim light indication (instead of a completely extinguished indication) that is in contrast with a bright light indication. These dim and bright light indications represent the two conditions of the transistor (or the two voltage levels being indicated). Similarly, for other firing and extinction potential variations `than those indicated in Figure 4, the peak-to-peak A.C. voltage amplitude may be so chosen that the tube 40 would be completely extinguished for each A.C. cycle to represent one input voltage level, and tired for say 1% of an A.C. cycle to represent the other input voltage level. Such increase of the A.C. voltage amplitude permits a greater range of variation in tube characteristics at the sacrifice of the amount of difference in light levels that is used to provide the indications desired.

The circuit of Figure 3 may be used with various transistor circuits to indicate voltage variations at other transistor electrodes.

In Figure 5, a bistable transistor ip-op is shown, the two stable states of which are indicated by a neon tube circuit of the tvpe described above with respect to Figure 3. The flip-flop includes two transistors 90 and 92 that are cross-coupled in a well known manner from collector to base by means of resistor-capacitor combinations. These transistors 90 and 92 may be set and reset respectively by negative going pulses applied to their bases. K

Connected to the collector of the transistor 90 is an indicating circuit of the type described above; namely, the series combination of a limiting resistor 94, a neon tube 96, and a D.C. and A.C. power supply that includes av transformer 98. A diode 100 is used to clamp the collector of the transistor 90 to a suitable voltage level,

shown in Figure 5 as -12 volts. The emitter of the` transistor 90 is returned to a reference level of +6 volts, which condition results in a voltage swing at the collector of the transistor 90 between -12 volts and +6 volts for the transistor being in the conductive and nonconductive states respectively. Assuming the same range of variations in neon tube characteristics as those discussed above with respect to Figures 3 and 4, a 12 volt peak-to-peak A.C. voltage amplitude is provided by the transformer 98. The D.C. bias voltage level is 64 volts (instead of the 58 volts shown in Figure 3) because the D.C. level of the collector voltage is 6 volts higher in condition, which results in a collector voltage of |ubstantially +6 volts. As a result, the voltage across the tube 96 is reduced to below the extinction potential, and the tube 96 is extinguished. Thus, lighted and extinguished conditions of the tube 96 respectively indicate set and reset conditions of the ip-op circuit.

Such indicator circuits may be used to indicate the states of a plurality of similar llip-ops connected, for example, as a binary counter. The tubes 46 of each such indicator may be connected to but a single A.C. and D.C. power supply; that is, the transformer 98 and +64 volt D.C. source would be common to all the indicators.

By means of this invention, a new and improved indicator circuit is provided by means of gas discharge tubes. Such tubes may be used for indicating relatively small voltage changes, and reliable operation may be provided notwithstanding variations in tube characteristics.

What is claimed is:

1. In combination, a device having a turn-on voltage of a first value and a turn-o voltage of a second value; means for applying an alternating voltageto said device having a peak-to-peak amplitude not greater than the difference between said first and second values; and means for applying a direct voltage to said device which can be switched between a first level at which one peak of said alternating voltage exceeds said turn-on voltage and a second level at which the other peak of said alternating voltage is less than said turn-oit voltage.

2. In the combination as set forth in claim l, said 1astnamed means including a direct bias voltage source and a source providing a direct voltage having one of two discrete Values, the difference between said values being only a fraction of the diterence between said turn-on and turn-o voltages.

3. In combination, a gas discharge device having solely two electrodes, said device having a tiring voltage of one value and an extinction voltage of another, substantially lower value; means for applying across said electrodes an alternating voltage having a peak-to-peak amplitude substantially equal to the diierence between said firing and extinction voltages; and means for applying in series With said alternating voltage a control voltage which can be switched between a first level at which one peak of said alternating voltage exceeds said tiring voltage and a second level at which the opposite peak of said alternating voltage is less than said extinction voltage.

4. In combination, a gas discharge device having sole- 1y two electrodes, said device having a tiring voltage of one value and an extinction voltage of another, substantially lower value; means for applying a direct biasing voltage across said electrodes having a value substantially half-way between the ring and extinction voltages of saiid discharge device; means for applying in series with said biasing voltage an alternating voltage having a peak-to` peak amplitude not greater than the diierence between said tiring and extinction voltages; and means for applying in series with said biasing voltage a direct voltage having one of two discrete levels, said levels being such that at one level one peak of the alternating voltage tires said discharge device, and at said other level the opposite peak of the alternating voltage extinguishes said discharge device.

References Cited in the le of this patent UNITED STATES PATENTS 2,069,915 Edwards et al Feb. 9, 1937 2,128,395 Berndt et al Aug. 30, 1938 2,246,046 Holden June 17, 1941 2,506,723 Larsen May 9, 1950 

